Transient absorption spectroscopy (TAS) is a powerful non-linear technique for
investigating ultrafast state-resolved processes and electronic superposition
using two pulses, viz pump and time-delayed probe
pulse. [1]}, [2]}, [3]} TAS offers additional degrees
of freedom, namely, pump field features like shape, amplitude, carrier
frequency, direction, and pump-probe time delays, absent in conventional
spectroscopy. Of which, the effect of time-delay between the pulses is most
commonly followed experimentally. [4]}, [5]}
Since long, the emphasis has been on the femto- to pico-second timescales,
which involved time-resolving processes initiated by nuclear
motions. [6]}, [7]}, [8]}, [9]}
However, with the advent of atto-second pulses, there has been an ever
increasing interest in studying sub-femtosecond timescale processes, focusing
on the motion of electrons on their natural time-scale, with minimal influence of
nuclear degrees of
freedom. [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [2]}, [19]}, [20]}
Simultaneously, generation of intense isolated soft X-ray free electron laser
pulses with sub-femtosecond temporal widths has been achieved recently,
promoting investigations of attosecond resolved spectroscopies involving core
orbitals. [21]}, [22]} Due to their
characteristic element specificity and unprecedented temporal resolution,
ultrafast X-ray spectroscopies are emerging as indispensable tools for
biological and material sciences. [23]}, [24]}
Especially X-ray transient absorption spectroscopy (XTAS) which employs short
X-ray probe pulses is now extensively being used to investigate electron
dynamics of molecules and solid-state
systems. [25]}, [23]}, [7]}, [28]}, [29]}, [24]}, [31]}
This dictates the need to develop reliable theoretical tools to aid the
interpretation and prediction of such phenomena.
